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In your mathematics class, you likely studied geometry, examining different shapes and the characteristics of those shapes, such as area, perimeter, and other factors. The geometric objects in ggplot2 are visual structures that are used to visualize data. They can be lines, bars, points, and so on.
Geometric objects are constructed from datasets. Before we construct some geometric objects, let's examine some datasets to understand the different kinds of variables.
We all love to talk about the weather. So, let's work with some weather-related datasets. The datasets contain approximately five years' worth of high-temporal resolution (hourly measurements) data for various weather attributes, such as temperature, humidity, air pressure, and so on. We'll analyze and compare the humidity and weather datasets.
Let's begin by implementing the following steps:
df_hum <- read.csv("data/historical-hourly-weather-data/humidity.csv")
df_desc <- read.csv("data/historical-hourly-weather-data/weather_description.csv")
The outcome will be the humidity levels of different cities, as follows:
The weather descriptions of different cities are shown as follows:
The different geometric objects that we will be working with in this chapter are as follows:
One-dimensional objects are used to understand and visualize the characteristics of a single variable, as follows:
Two-dimensional objects are used to visualize the relationship between two variables, as follows:
Although geometric objects are also used in base R, they don't follow the structure of the Grammar of Graphics and have different naming conventions, as compared to ggplot2. This is an important distinction, which we will look at in detail later.
Histograms are used to group and represent numerical (continuous) variables. For example, you may want to know the distribution of voters' ages in an election. A histogram is often confused with a bar chart; however, a bar chart is more general, and we will cover those later. In a histogram, a continuous variable is grouped into bins of specific sizes and the bins have a range that covers the maximum and minimum of the variable in question.
Histograms can be classified as follows:
Let's take a look at another image:
It's important to study the shapes of distributions, as they can reveal a lot about the nature of data. We will see some real-world examples of histograms in the datasets that we will explore.
We discussed the different kinds of geometric objects that we will be working on, and we created our fist plot using two different methods (qplot and hist). Now, let's use another command: ggplot. We will use the humidity data that we loaded previously.
As seen in the preceding section, we can create a default histogram by using one of the commands in the base R package: hist. This is seen in the following command:
hist(df_hum$Vancouver)
The default histogram that will be created is as follows:
In this section, we want to visualize the humidity distribution for the city of Vancouver. We'll create a histogram for humidity data using qplot and ggplot.
Let's begin by implementing the following steps:
ggplot(df_hum,aes(x=Vancouver))
Notice that there are some warning messages, as follows:
'stat_bin()' using 'bins = 30'. Pick better value with 'binwidth'.
Warning message:
Removed 1826 rows containing non-finite values (stat_bin).
You can ignore these messages; ggplot automatically detects and removes null or NA values.
ggplot (df_hum, aes(x=Vancouver)) + geom_histogram()
You'll see the following output:
Here's the output code:
require("ggplot2")
require("tibble")
#Load a data file - Read the Humidity Data
df_hum <- read.csv("data/historical-hourly-weather-data/humidity.csv")
#Display the summary
str(df_hum)
qplot(df_hum$Vancouver)
ggplot(df_hum, aes(x=Vancouver)) + geom_histogram()
Scenario
Histograms are useful when you want to find the peak and spread in a distribution. For example, suppose that a company wants to see what its client age distribution looks like. A two-dimensional distribution can show relationships; for example, one can create a scatter plot of the incomes and ages of credit card holders.
Aim
To create and analyze histograms for the given dataset.
Prerequisites
You should be able to use ggplot2 to create a histogram.
Steps for Completion
Outcome
Two histograms should be created and compared. The complete code is as follows:
df_t <- read.csv("data/historical-hourly-weather-data/temperature.csv")
ggplot(df_t,aes(x=Vancouver))+geom_histogram()
ggplot(df_t,aes(x=Miami))+geom_histogram()
Take a look at the following output histogram:
From the preceding plot, we can determine the following information:
Take a look at the following output histogram:
From the preceding plot, we can determine the following information:
Differences
Bar charts are more general than histograms, and they can represent both discrete and continuous data. They can even be used to represent categorical variables. A bar chart uses a horizontal or vertical rectangular bar that levels of at an appropriate level. A bar chart can be used to represent various quantities, such as frequency counts and percentages.
We will use the weather description data to create a bar chart. To create a bar chart, the geometric object used is geom_bar().
The syntax is as follows:
ggplot(….) + geom_bar(…)
If we use the glimpse or str command to view the weather data, we will get the following results:
Use the ggplot(df_vanc,aes(x=Vancouver)) + geom_bar() command to obtain the following chart:
Observations
Vancouver has clear weather, for the most part. It rained about 10,000 times for the dataset provided. Snowy periods are much less frequent.
We will now perform two exercises, creating a one-dimensional bar chart and a two-dimensional bar chart. A one-dimensional bar chart can give us the counts or frequency of a given variable. A two-dimensional bar chart can give us the
relationship between the variables.
In this section, we'll count the number of times each type of weather occurs in Seattle and compare it to Vancouver.
Let's begin by following these steps:
ggplot(df_vanc,aes(x=Seattle)) + geom_bar()
You should see the following output:
Answers
It rained on approximately 40% of the days.
A two-dimensional bar chart can be used to plot the sum of a continuous variable versus a categorical or discrete variable. For example, you might want to plot the total amount of rainfall in different weather conditions, or the total amount of sales in different months.
In this section, we'll create a two-dimensional bar chart for the total sales of a company in different months.
Let's begin by following these steps:
ggplot(RetailSales,aes(x=Month,y=Sales)) + geom_bar(stat="identity")
A screenshot of the expected outcome is as follows:
A boxplot (also known as a box and whisker diagram) is a standard way of displaying the distribution of data based on a file-number summary: minimum, first quartile, median, third quartile, and maximum. Boxplots can represent how a continuous variable is distributed for different categories; one of the axes will be a categorical variable, while the other will be a continuous variable. In the default boxplot, the central rectangle spans the first quartile to the third quartile (called the interquartile range, or IQR). A segment inside of the rectangle shows the median, and the lines (whiskers) above and below the box indicate the locations of the minimum and maximum, as shown in the following diagram:
The upper whisker extends from the hinge to the largest and smallest values of ± 1.5 * IQR from the hinge. Here, we can see the humidity data as a function of the month. Data beyond the end of the whiskers are called outliers, and are represented as circles, as seen in the following chart:
You'll get the preceding chart by using the following code:
ggplot(df_hum,aes(x=month,y=Vancouver)) + geom_boxplot()
In this section, we'll create a boxplot for monthly temperature data for Seattle and San Francisco, and compare the two (given two points).
Let's begin by implementing the following steps:
The following observations can be noted:
The humidity is more uniform for San Francisco:
The median humidity for San Francisco is about 75:
A scatter plot shows the relationship between two continuous variables. Let's create a scatter plot of distance versus time for a car that is accelerating and traveling with an initial velocity. We will generate some random time points according to a function. The relationship between distance and time for a speeding car is as follows:
We can draw a scatter plot to show the relationship between distance and time with the following code:
ggplot(df,aes(x=time,y=distance)) + geom_point()
We can see a positive correlation, meaning that as time increases, distance increases. Take a look at the following code:
a=3.4
v0=27
time <- runif(50, min=0, max=200)
distance <- sapply(time, function(x) v0*x + 0.5*a*x^2)
df <- data.frame(time,distance)
ggplot(df,aes(x=time,y=distance)) + geom_point()
The outcome is a positive correlation: as time increases, distance increases:
The correlation can also be zero (for no relationship) or negative (as x increases, y decreases).
A line chart shows the relationship between two variables; it is similar to a scatter plot, but the points are connected by line segments. One difference between the usage of a scatter plot and a line chart is that, typically, it's more meaningful to use the line chart if the variable being plotted on the x-axis has a one-to-one relationship with the variable being plotted on the y-axis. A line chart should be used when you have enough data points, so that a smooth line is meaningful to see a functional dependence:
We could have also used a line chart for the previous plot. The advantage of using a line chart is that the discrete nature goes away and you can see trends more easily, while the functional form is more effectively visualized.
If there is more than one y value for a given x, the data needs to be grouped by the x value; then, one can show the features of interest from the grouped data, such as the mean, median, maximum, minimum, and so on. We will use grouping in the next section.
In this section, we'll create a line chart to plot the mean humidity against the month. Lets's begin by implementing the following steps:
df_hum$monthn <- as.numeric(df_hum$month)
gp1 <- group_by(df_hum,monthn)
The following plots are obtained:
Take a look at the output line chart:
Scenario
Suppose that we are in a company, and we have been given an unknown dataset and would like to create similar plots. For example, we have some educational data, and we would like to know what courses are the most popular, or the gender distribution among students, or how satisfied the parents/students are with the courses. We will use the new dataset, along with our own knowledge, to get some information on the preceding points.
Aim
To create one- and two-dimensional visualizations for the new dataset and the given variables.
Steps for Completion
Outcome
Three one-dimensional plots and three two-dimensional plots should be created, with the following axes (count versus topic) and observations. (Note that the students may provide different observations, so the instructor should verify the answers. The following observations are just examples.)
This visual was chosen because Topic is a categorical variable, and I wanted to see the frequency of each topic:
Observation
You can see that IT is the most popular subject:
gender is a categorical variable; you can chose a bar chart because you wanted to see the frequency of each topic.
Observation
You can observe that more males are registered in this institute from the following histogram:
VisitedResources is numerical, so you can choose a histogram to visualize it.
Observation
It's a bimodal histogram with two peaks, around 12 and 85.
Take a look at the following 2D plots:
Plot 1:
Plot 2:
Plot 3:
Observations
It is also possible to plot using three-dimensional vectors. This creates a three-dimensional plot, which provides enhanced visualization for applications (for example, displaying three-dimensional spaces). Essentially, it is a graph of two functions, embedded into a three-dimensional environment.
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